1. Field of the Invention
The present invention relates to electronic displays, and more particularly to a photonic crystal LED-based (light emitting diode-based) surface light source for an LCD (liquid-crystal display).
2. Prior Art
In general, LCDs have two main advantages in comparison with cathode ray tubes (CRTs): LCDs are thin, and have low power consumption. It has been said that LCDs might one day completely replace CRT display devices, and LCDs have aroused great interest in many industries in recent times. In general, an LCD needs a surface light source to provide uniform light for a clear display.
A surface light source comprises a light source and a light guide plate. The light source may be an LED, or one or more fluorescent lamps. Compared to conventional backlights that use fluorescent lamps, LEDs are less expensive, last longer, contribute to a more robust design, and provide a higher brightness-to-size ratio. The light guide plate has an end face through which light is introduced, and two opposite major faces one of which functions as an emission face. The performance of the surface light source greatly depends on the characteristics of the light guide plate employed therein.
A light guide plate functions to change a direction of propagation of light beams emitted from the light source and introduced into the light guide plate, from a direction roughly parallel to the emission face of the light guide plate to a direction perpendicular to the emission face. That is, the light guide plate effectively changes the linear or point light source(s) into a surface light source, for evenly illuminating a whole display screen of the LCD.
FIG. 7 shows a conventional LED 110, which includes a substrate 111, a first electrode 112, a second electrode 115, a wire 116, an LED chip 113, and an encapsulating resin 114. The LED chip 113 is mounted on the first electrode 112 by a dielectric glue, and electrically connects with the second electrode 115 through the wire 116. The encapsulating resin 114 covers the LED chip 113, for protecting the LED chip 113 from water vapor. The LED chip 113 has abase layer 1131 and a film layer 1132. The LED chip 113 further has a plurality of emitting modes, in which light beams are emitted from different emitting directions. Therefore, the LED chip 113 produces total internal reflection and diffusing, which decreases the extraction efficiency and emitting intensity of the LED 110.
FIG. 8 shows a conventional surface light source 10, which comprises a reflector 130, a light guide plate 120, a diffuser 140, a brightness enhancing film (BEF) 150, two point light sources 110, and two light source covers 160 partially covering the light sources 110 respectively. The reflector 130, the light guide plate 120, the diffuser 140 and the BEF 150 are stamped together from bottom to top in that order. The light guide plate 120 comprises a top light emitting surface 122, a bottom surface 123, and a pair of opposite incident surfaces 121. The point light sources 110 are disposed adjacent the incident surfaces 121 respectively. In use, light beams from the point light sources 110 enter the light guide plate 120 through the incident surfaces 121, are emitted from the light emitting surface 122, and then are introduced into the BEF 150 in a direction perpendicular thereto. The BEF 15 converges the light beams in a predetermined direction to evenly illuminate a whole display screen of an associated LCD. Each point light source 110 is the LED of FIG. 7 described above.
In use, the LED 110 has a relatively low emitting intensity, which decreases the optical characteristics of the surface light source 10 and the associated liquid crystal display. In addition, the surface light source 1 generally needs to comprise all the above-described elements, including the diffuser 14 and the BEF 15, for enhancing brightness. This adds to the size, the cost, and the difficulty of the manufacturing the surface light source 10.
A new surface light source for an LCD which overcomes the above-mentioned disadvantages is desired. In particular, what is needed is an LED having a high extraction efficiency, and a surface light source utilizing the LED for high illumination.